IL17A and inflammatory bowel disease: Accumulating evidence has demonstrated that CD4+ T cells are critical for the development of chronic intestinal inflammation, and CD4+ T-cell-related cytokines, such as tumor necrosis factor (TNF), interferon (IFN)-γ, interleukin (IL)-12, and IL-17A, are up-regulated in the inflamed mucosa of patients with IBD (5–7).